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Sun, 07 Dec 2025 18:32:41 -0800 (PST) From: Lance Yang To: david@kernel.org Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, aneesh.kumar@kernel.org, arnd@arndb.de, harry.yoo@oracle.com, jannh@google.com, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, liushixin2@huawei.com, loberman@redhat.com, lorenzo.stoakes@oracle.com, muchun.song@linux.dev, nadav.amit@gmail.com, npiggin@gmail.com, osalvador@suse.de, peterz@infradead.org, pfalcato@suse.de, prakash.sangappa@oracle.com, riel@surriel.com, stable@vger.kernel.org, vbabka@suse.cz, will@kernel.org, Lance Yang Subject: Re: [PATCH v1 1/4] mm/hugetlb: fix hugetlb_pmd_shared() Date: Mon, 8 Dec 2025 10:32:31 +0800 Message-ID: <20251208023231.1257-1-ioworker0@gmail.com> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20251205213558.2980480-2-david@kernel.org> References: <20251205213558.2980480-2-david@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 3AD26C000B X-Stat-Signature: xbmje7kpeefbymhigrsxi5m7ambdaow3 X-Rspam-User: X-HE-Tag: 1765161163-319009 X-HE-Meta: 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 qGC/RKC8 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Lance Yang On Fri, 5 Dec 2025 22:35:55 +0100, David Hildenbrand (Red Hat) wrote: > We switched from (wrongly) using the page count to an independent > shared count. Now, shared page tables have a refcount of 1 (excluding > speculative references) and instead use ptdesc->pt_share_count to > identify sharing. > > We didn't convert hugetlb_pmd_shared(), so right now, we would never > detect a shared PMD table as such, because sharing/unsharing no longer > touches the refcount of a PMD table. > > Page migration, like mbind() or migrate_pages() would allow for migrating > folios mapped into such shared PMD tables, even though the folios are > not exclusive. In smaps we would account them as "private" although they > are "shared", and we would be wrongly setting the PM_MMAP_EXCLUSIVE in the > pagemap interface. > > Fix it by properly using ptdesc_pmd_is_shared() in hugetlb_pmd_shared(). > > Fixes: 59d9094df3d7 ("mm: hugetlb: independent PMD page table shared count") > Cc: > Cc: Liu Shixin > Signed-off-by: David Hildenbrand (Red Hat) > --- Tested on x86 with two independent processes sharing a 1GiB hugetlbfs file (aligned a 1GiB boundary). Before the fix, even though PMD sharing worked (pt_share_count=1), hugetlb_pmd_shared() returned false because page_count() was still 1, causing smaps to report it as "Private" and pagemap to set it PM_MMAP_EXCLUSIVE incorrectly :( After the fix, hugetlb_pmd_shared() correctly detects the sharing, smaps reports it as "Shared", and PM_MMAP_EXCLUSIVE is cleared ;) Tested-by: Lance Yang Cheers! > include/linux/hugetlb.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h > index 019a1c5281e4e..03c8725efa289 100644 > --- a/include/linux/hugetlb.h > +++ b/include/linux/hugetlb.h > @@ -1326,7 +1326,7 @@ static inline __init void hugetlb_cma_reserve(int order) > #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING > static inline bool hugetlb_pmd_shared(pte_t *pte) > { > - return page_count(virt_to_page(pte)) > 1; > + return ptdesc_pmd_is_shared(virt_to_ptdesc(pte)); > } > #else > static inline bool hugetlb_pmd_shared(pte_t *pte)